This invention relates generally to a multi-channel communication system and more particularly to a multi-channel delta modulation system.
The transmission of analog signals by conversion to digits is well known. In multi-channel systems, economic use of the transmission medium is afforded by employing time division multiplexing of the digital signals.
One method of converting an analog signal to digital form is by employing pulse code modulation. In such systems, the source signal is periodically sampled and the sequence of analog samples is applied to an analog-to-digital converter which generates a representative digital code word for each sample. After transmission, the digital code words are converted to analog samples by applying them to a suitable digital-to-analog converter. Where there are a plurality of analog signal sources, they may be sequentially and periodically sampled to form a time division multiplex of analog signals which is applied to a single analog-to-digital converter to form a time division multiplexed digital signal for transmission. At the receiving end, the multiplexed digital signal is applied to a single digital-to-analog converter. The output of the digital-to-analog converter is applied to an analog demultiplexer which serves to sequentially and periodically distribute the reconstructed analog samples to the corresponding signal sinks.
Another method of converting an analog signal to digital form is by employing delta modulation. In present delta modulation systems, the source signal is periodically compared to an approximating signal stored in a local accumulator. The comparison generates logic 1 or logic 0 according to whether the source signal is greater or less than the accumulator signal. After each comparison, the accumulator signal is incremented one step positively or negatively according to whether the comparison yielded logic 1 or logic 0, thereby providing an up-dated approximation signal for the next comparison. By this mechanism, the local accumulator signal is forced to be a stepped approximation to the source signal. The digital signal generated by the comparison is also transmitted to a receiving accumulator which is identical to the local accumulator. Therefore, the output signal of the receiving accumulator is also a stepped approximation to the source signal.
In the delta modulation process, it is desirable to use a small accumulator step size in order to obtain a close approximation to the source signal. But the maximum rate at which the approximating signal can change is the product of accumulator step size and channel bit rate. If the source signal changes faster than this rate, the accumulators cannot follow closely and the approximating signal is said to be distorted by "slope overload." A delta modulation process in which the accumulator step size is maintained constant is known as linear delta modulation. For many signals (e.g. telephone speech), the linear delta modulation process requires unreasonably high bit rates to avoid excessive slope overload distortion.
This disadvantage has been overcome by the development of "adaptive delta modulation" in which the accumulator step size is dynamically adjusted in accordance with the slope of the source signal, or in accordance with this slope as inferred from observations of the digital signal generated by the comparator. The transmission quality of an adaptive delta modulation channel is comparable to that of a pulse code modulation channel operating at the same bit rate.
In a multi-channel delta modulation communication system in accordance with the prior art, an encoder and decoder is provided for each channel. The digital outputs of the encoders are multiplexed for transmission and demultiplexed upon reception and applied to individual decoders. In view of the fact that there is an encoder and a decoder for each channel, a multi-channel system is relatively expensive even if the individual encoders and decoders are only moderately expensive. Conversely, as previously described, a multi-channel pulse code modulation system requires only one coder and one decoder time shared by all the channels. Consequently, multi-channel pulse code modulation has been more economical than multi-channel delta modulation.